The film concerns the coming destruction of the Earth by the rogue star Bellus and the desperate efforts to build a space ark that will save and transport a small portion of humanity to the star's single orbiting planet, Zyra.

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Pilot David Randall is paid to fly top-secret photographs from South African astronomer Dr. Emery Bronson to Dr. Cole Hendron in the United States. Hendron, with the assistance of his daughter Joyce, confirms their worst fears: Bronson has discovered a rogue star named Bellus that is on a collision course with Earth.

Hendron warns the delegates of the United Nations that the end of the world is little more than eight months away. He pleads for the construction of spaceships to transport a lucky few to Zyra, the sole planet in orbit around Bellus, in the faint hope that it can sustain life and save the human race from extinction. However, other equally distinguished scientists scoff at his claims, and he is not believed by the UN delegates.

Hendron receives help from wealthy humanitarian friends, who arrange a lease on a former proving ground to construct an ark spaceship. To finance the construction, Hendron is forced to accept money from the self-centered, wheelchair-bound industrialist Sidney Stanton. Stanton demands the right to select the passengers, but Hendron insists that he is not qualified to make those choices; all he can buy with his wealth is a single seat aboard the ark.

Joyce becomes attracted to Randall and prods her father into finding reasons to keep him around, much to the annoyance of her boyfriend, medical doctor Tony Drake. The ship's construction is a race against time. As Bellus nears, former skeptics admit that Hendron was right and governments prepare for the inevitable. Groups in other nations also begin building spaceships. Martial law is declared and residents in coastal regions are moved to inland cities.

When Zyra makes a close approach, its gravitational attraction causes massive earthquakes, volcanic eruptions, and tidal waves that wreak havoc around the world. Several people are killed at the ark's construction camp, including Dr. Bronson. In the aftermath, Drake and Randall travel by helicopter to provide assistance to survivors. When Randall leaves the helicopter to rescue a little boy stranded on a rooftop surrounded by water, Drake must resist a strong temptation to strand his rival.

As the day of doom approaches, the spaceship is loaded with food, medicine, microfiche copies of books, equipment, and animals. Finally, the lucky passengers are selected by a lottery, though Hendron reserves seats for a handful of people: himself, Stanton, Joyce, Drake, pilot Dr. George Frey, the young boy who was rescued, and Randall, for his daughter's sake. When a young man turns in his winning ticket because his sweetheart was not selected, Hendron arranges for both to go. Randall refuses his seat and only pretends to participate in the lottery, believing he has no skills needed for settling on Zyra. For Joyce's sake, Drake fabricates a "heart condition" for Frey, making a co-pilot necessary; Randall is the only choice.

The cynical Stanton becomes increasingly anxious as time passes. Knowing human nature, he fears what the desperate lottery losers might do, so as a precaution, he has stockpiled weapons; Stanton's suspicions prove to be well-founded. His much-abused assistant, Ferris, tries to add himself at gunpoint to the passenger manifest, only to be shot dead by Stanton. During their final night on Earth, the selected passengers and animals are quietly moved to the launch pad to protect them from more violence.

Shortly before takeoff, many of the lottery losers riot, taking up Stanton's weapons to try to force their way aboard the space ark. Hendron surprises everyone by staying behind at the last moment and forcibly keeps Stanton with him in order to conserve fuel for the flight. With an effort born of ultimate desperation, Stanton stands up and starts to walk in a futile attempt to board the departing spaceship.

The crew are rendered unconscious by the g-force of acceleration and do not witness the Earth's collision with Bellus, displayed on the forward television monitor. When Randall comes to and sees Dr. Frey already awake and piloting the ship, he realizes he has been deceived.

As the space ark enters Zyra's atmosphere, the fuel finally runs out; Randall takes control, gliding the ship to a rough, but safe landing. Earth's survivors begin to disembark, finding Zyra to be habitable. David Randall and Joyce Hendron follow, walking hand-in-hand down the ramp to explore an unknown but hopeful future.

A feature film, based on the original novels When Worlds Collide and After Worlds Collide, first serialized in Blue Book magazine in 1932, was first considered for filming by Cecil B. Demille in the 1930s. When George Pal began producing his version years later, he initially wanted a more lavish production with a larger budget, but he wound up being forced to scale back his plans.[3]

Douglas Fairbanks Jr. was first considered for the role of Dave Randall, but Richard Derr was finally hired for the part.[4]

Chesley Bonestell is credited with the artwork used for the film; he created the design for the space ark that was constructed to journey to Zyra. The final scene in the film, showing the sunrise landscape of the alien world, was taken from a Bonestell sketch. Because of budget constraints, the director was forced to use this color sketch rather than a finished matte painting, drawing criticism. The additional poor quality still image showing a drowned New York City is often attributed to Bonestell, but it was not actually drawn by him.[5]

The Differential analyzer at UCLA is shown briefly near the beginning of the film; it verifies the initial hand-made calculations confirming the coming destruction of the Earth. "There is no error."[3]

Producer George Pal considered making a sequel based on the second novel After Worlds Collide, but the box office failure of his 1955 Conquest of Space made that impossible.[3]

When Worlds Collide was reviewed by Bosley Crowther of The New York Times noted that George Pal had followed up on his other prophetic epic. Destination Moon: "... this time the science soothsayer, whose forecasts have the virtue, at least, of being represented in provocative visual terms, offers rather cold comfort for those scholars who would string along with him. One of the worlds which he arranged to have collide is ours."[6]

Freelance writer Melvin E. Matthews calls the film a "doomsday parable for the nuclear age of the '50s."[7]Emory University physics professor Sidney Perkowitz notes that this film is the first in a long list of movies where "science wielded by a heroic scientist confronts a catastrophe." He calls the special effects exceptional.[8]

Librarian and filmographer Charles P. Mitchell was critical of the "... scientific gaffes that dilute the storyline," as well as a "failure to provide consistent first class effects." He pointed out that there were inconsistencies in the script, such as the disappearance of Dr. Bronson in the second half of the film.[Note 1] In his flawed analysis, Mitchell also does not recognize that sister spacecraft are being built by other nations and their ultimate fate. He summarizes by saying, "the large number of plot defects are annoying and prevent this admirable effort from achieving top-drawer status."[4]

In the film adaptation of L.A. Confidential (1997), tabloid writer Sid Hudgens arranges for the publicity-loving Jack Vincennes to arrest a young actor on the night of this film's premiere, resulting in photos of the arrest with the theatre holding the premiere in the background accompanied by the headline "Movie Premiere Pot Bust" (the scene is shown taking place in 1953, long after the 1951 premiere of When Worlds Collide).[12]

Paramount Pictures began pre-production on a remake of When Worlds Collide c.2013. As of April 2015, however, its status remained unknown, according to IMDB, and it appears unlikely that the remake will ever be made.[15]